This invention relates generally to linear transmitters and, more particularly, to a linear transmitter incorporating an isolator elimination element for maintaining stability of a feedback loop.
Radio communication devices use antennas to provide for the efficient transmission of radio frequency (RF) communication signals. The transmitter portion of a radio communication device includes a power amplifier to amplify the RF signals before they are coupled to the antenna for transmission. For some modulation techniques, such as quadrature amplitude modulation (QAM), linear amplification is desired to prevent distortion of the modulated signal. However, when RF power amplifiers are operated in their most efficient manner at high drive levels, they usually provide a non-linear xe2x80x9ccompressionxe2x80x9d characteristic. This means that a change in the amplitude of a signal sent into the power amplifier results in a non-proportional change in the amplitude of the signal out of the amplifier, and therefore distortion. Non-linearities may also be caused by changes in load impedance, as may be caused by the operating environment of the power amplifier.
One manner of improving the linearity of an RF transmitter is to use a Cartesian feedback loop, whereby a feedback signal path is provided to create a negative feedback which compensates for compression in the power amplifier. In so doing, the level of energy transmitted on adjacent channel frequencies, known as splatter, is reduced. A component known as an isolator or circulator is sometimes inserted between the power amplifier and antenna in an effort to prevent changes in the power amplifier load from causing clipping and distortion, or from causing phase shifts that can make the Cartesian loop unstable. Although the isolator works well in some applications, it adds significant cost, size and weight to the design of a radio communication device.
Accordingly, it is desirable to provide Cartesian feedback linear transmitters that do not include isolators, yet are capable providing a stable and linear response without clipping and distortion. One such transmitter is described and claimed in the referenced U.S. Pat. No. 5,675,286 (xe2x80x9cthe ""286 patentxe2x80x9d). The transmitter of the ""286 patent includes a circuit, termed an xe2x80x9cisolator eliminatorxe2x80x9d circuit that provides for the elimination of the isolator by continuously tracking and correcting loop gain, phase and level set changes for the Cartesian feedback loop. The present invention discloses a transmitter incorporating an alternative form and manner of isolator elimination, but which otherwise has similar functionality as that of the ""286 patent. Advantageously, the isolator eliminator of the present invention is implemented in software, executable by a processor such as a digital signal processor (DSP) to provide flexibility to accommodate multiple protocols including, but not limited to, integrated dispatch enhanced network (iDEN), Association of Public Safety Communication Officials Project 25 (APCO-25), terrestrial trunked radio (TETRA), global system for mobile communication (GSM), and Enhanced Data Rate for Global Evolution (EDGE).